Energy production is always racing toward the next wave of technologies that will produce cleaner, more efficient, and less expensive energy. Fuel cells are often considered one of the most efficient procedures for generating energy. The high efficiency of fuel cells and their low level of emissive pollutants justify the enormous effort made during the last years for continued research and practical implementation. But one of the major difficulties for commercially implementing fuel cells is the clean production and handling of hydrogen required for fuel cell operation.
The use of hydrogen as an energy carrier is environmentally attractive because it can be burned cleanly in a manner that only produces water (H2O) and carbon dioxide (CO2). Gasoline, diesel, and other fuels derived from petroleum produce much more harmful emissions. Yet, hydrogen only occurs in nature in combination with other elements, such as with oxygen in water and with carbon in hydrocarbons (e.g., propane, ethanol, etc.). Molecular hydrogen can be used as an energy carrier once it has been broken from the bonded state and combined with other elements. Unfortunately, doing so requires significant energy.
The proton exchange membrane (PEM) is among the more developed systems used in today's fuel cells. A major advantage of the PEM fuel cell is its efficiency in utilization of the fuel energy content versus, for example, the internal combustion engine. The ideal fuel for current PEM fuel cells is hydrogen. There are two well established technologies for producing pure hydrogen: a steam methane reforming process and the electrolysis of water.
Steam methane reforming is the most common process for producing hydrogen commercially. Modern steam methane reforming units produce hydrogen in a four step process. First, natural gas is processed in a pre-treat step with hydrogen to remove sulfur. Second, methane (or other hydrocarbon) mixed with steam is passed over a nickel oxide catalyst at temperatures of 700°-1000° C. and at nearly 30 atm to produce hydrogen and carbon monoxide (CO). This reaction is highly endothermic, requiring a substantial amount of heat. Third, the hydrogen and carbon monoxide are supplied to a water-gas shift reactor that adds water to produce additional hydrogen and carbon dioxide. This reaction is exothermic and is readily carried out between 200°-350° C. Fourth, the hydrogen is then purified before being sent to other devices for use. The entire process requires a large amount of heat and ends up generating considerable amounts of poisonous carbon monoxide.
Hydrogen can also be produced through the electrolysis of water. The cost of producing hydrogen by electrolysis via current electrolytic processes is largely dependent on the cost of electricity, the efficiency of the process, and the capital costs of the systems involved. Today's electrolysis systems are highly inefficient, and the energy required to produce hydrogen is expensive. A reduction in the system capital costs and improvement in system efficiency are needed to make electrolysis more competitive for widespread use.